The present invention relates to a method for manufacturing a semiconductor device, in which electric components are formed on a thin semiconductor layer of an SOI substrate, and to an ultrathin semiconductor device. In particular, it relates to a semiconductor device manufacturing method, which allows to form electric components on a very thin semiconductor layer, such as an SOI structure, without causing neither electric component destruction nor damage of oxidized layer due to the charging up by the plasma in the manufacturing process, and the present invention relates also to an ultrathin semiconductor device.
Conventionally, semiconductor devices using SOI substrate have various advantages, such that the electric components can be isolated from each other without using p-n junctions, that the leak current is small and that the capacitance is small because electric components can be formed on a thin semiconductor layer disposed on an insulating layer, therefore, the operation speed of the semiconductor devices can be increased, and the degree of electric component integration can be increased. Thus, semiconductor devices using SOI substrate are used in high integrated semiconductor devices. Such semiconductor devices are manufactured using a wafer, which is prepared such that an isolating layer is formed between semiconductor layers as an SOI substrate.
Such an SOI substrate has been prepared by various methods, such as an combining method, oxygen implantation method, ELTRAN (Epitaxial Layer Transfer) method, and smart-cut method. In the combining (applying) method, two Si substrate are prepared; then an insulating layer, for example, SiO2, is formed on one of the two substrates; then they are applied to each other, and one of those Si substrates is ground to be thin. In the oxygen implantation method, oxygen ions are implanted up to a certain depth from the surface of a Si substrate so that an insulating layer, such as SiO2, is formed up to the depth from the surface.
In the ELTRAN method, a porous surface is formed on a Si substrate by an anode oxidization procedure, for example, then an epitxial semiconductor layer is grown on the porous surface. Another semiconductor substrate, on which an oxidized layer is formed, is combined (applied) on the surface of the epitaxial grown semiconductor layer of the former Si substrate, then the Si substrate portion is separated at the porous surface so that this portion is removed. And electric components are formed on the epitaxial grown semiconductor layer. In the smart-cut method, two substrates are prepared. Then, hydrogen ions are implanted into one of the substrates, and an oxidized layer is formed on the surface of the other substrate. Then they are applied to each other. After that, a heat treatment is undergone. The portion where hydrogen ions are implanted swells by the heat treatment so that the substrates are cut out at the portion.
In the process for manufacturing a semiconductor device using an SOI substrate, according to the prior art, as explained above, an SOI substrate is prepared previously before the so-called wafer process. Namely electric components are formed on a thin semiconductor layer, which is disposed on an insulating layer in the SOI substrate. This means that the thin semiconductor layer is electrically floating in the process, and the so-called charging up phenomenon occurs. The charging up phenomenon is a phenomenon that charges due to, for example, the plasma in the process of etching, CVD or ion implantation, accumulate in some portions. When an inverse electric field is formed by the accumulated charges, serious problems, such as destruction of the electric components or damages in the oxidized layer, will appear.
When such SOI substrate is not used, intrinsic gettering cites, which are called xe2x80x9cIG (intrinsic gettering)xe2x80x9d, are formed at a depth of over 20 micro meters from the surface of the semiconductor substrate 31, as shown in FIG. 5(a). The intrinsic gettering is a defect caused by precipitation of oxygen, and is also referred to xe2x80x9cbulk macro defect (BMD)xe2x80x9d. Impurities such as Fe, for example, or charges due to the implanted ions I/I are gettered by those gettering cites. When SOI substrate is used, however, the gettering cites IG are insulated by the oxidized layer 32, thus the gettering cites IG can not getter the impurities, etc. And thickness of the semiconductor layer 33 disposed on the oxidized layer 32 is very thin, for example, 0.05 to 2 micro meters, as shown in FIG. 5(b). Therefore no gettering cite cannot be formed therein. By the way, in FIGS. 5(a) and 5(b), reference numeral 34 denotes an electric component such as transistor and the reference numeral 35 denotes an insulating layer of the surface on the substrate 31.
According to the manufacturing method of a semiconductor device using SOI substrate, in the prior art, though the thickness of the semiconductor layer itself disposed on an insulating layer is formed to be thin, only a semiconductor device having a thickness of over 30 micro meters can be obtained. Because such semiconductor device is formed either on a Si substrate, on which an insulating layer is formed, or on a sapphire substrate, on which a semiconductor layer is formed. Thus, even when the silicon substrate is ground to be thin, there is a limit to decrease the thickness, due to the difficulty in the thinning of the substrate after forming electric components on such thick substrate. Thus, no semiconductor device thinner than 30 micro meters is obtained using such SOI substrate so far now. Another problem is that such SOI substrate is not suitable for putting on a plurality of such semiconductor devices one over another so as to produce a three dimensional semiconductor device.
An object of the present invention is to provide a manufacturing method of a semiconductor device using a SOI substrate, which allows to form electric components on a thin semiconductor substrate having SOI structure, while the electric components or the insulating layer are not influenced by the charging up phenomenon, which may appear in the manufacturing process.
Another object of the present invention is to provide an ultrathin semiconductor device having an insulating layer at its backside, the total thickness of which is under 20 micro meters, and which can be easily putted on (layered) one over another to form a three dimensional semiconductor device.
The method for manufacturing a semiconductor device is characterized by: (a) a step for forming a semiconductor substrate having a lacunose layer disposed at a equal depth from the surface of a semiconductor layer; (b) a step for forming electric components on the semiconductor layer at the surface side with respect to the lacunose layer; and (c) a step for separating the semiconductor substrate at the lacunose layer, after forming the electric components. An insulating layer is formed on the surface exposed by the separation of the semiconductor substrate at the lacunose layer, so that a semiconductor device having SOI structure is formed.
The lacunose layer in this description means a layer where the substrate can be easily separated into two parts. An example of a lacunose layer is, for example, a porous layer obtained by an anode oxidation of a silicon substrate. The substrate can be separated at the layer, by blasting a water jet or by N2 gas. Another example is a hydrogen ion implanted layer. The substrate can be separated at the layer, by generating foam due to a heat treatment. The electric component in this description means electric elements forming a part of an electric circuit, which are formed on and/or in the semiconductor layer. Examples of the electric component are transistor, memory element, diode, resistor, capacitor and lines.
According to this method, though electric components can be formed on a thin layer as if they are formed on a conventional SOI substrate, the electric components forming process can be carried out at a state that the semiconductor substrate is not electrically floating. Therefore, the electric component destruction or the damage of the insulating layer due to the plasma during the process does not occur, and charges or impurities can be gettered by gettering cites in the substrate so that high quality semiconductor devices can be produced. Further, an ultrathin semiconductor device having a total thickness of under 10 micro meters, the semiconductor device therein has an SOI structure, can be obtained, by disposing an insulating layer on the surface exposed by the separation of the semiconductor substrate at the lacunose layer in the backside thereof. Moreover a transistor having a very small serial resistance can be obtained as a discrete transistor, by disposing an electrode layer on the exposed surface of the semiconductor device, and by bonding directly the electrode with the substrate.
By the way, the insulating layer or the electrode layer can be disposed on the exposed surface as follows. After the completion of the electric components formation process, a supporting substrate is formed. The supporting substrate can be formed by applying a supporting tape for supporting the semiconductor layer or a glass plate, for example, quartz, on the front side of the semiconductor layer, or by covering the front side with poly-imide resin and curing it. Under the supporting by the supporting substrate, the semiconductor substrate is separated at the lacunose layer, then the insulating layers or the electrode layers are disposed on the exposed surface, subsequently. Finally the semiconductor devices are divided into chips, namely, an ultrathin semiconductor device can be obtained.
The semiconductor substrate having a lacunose layer, referred to in the step (a), can be prepared, for example, by making a porous surface on a silicon substrate by anode oxidation, and growing an epitxial semiconductor layer on the porous surface so that the porous portion becomes a lacunose layer, or by implanting hydrogen ions up to a certain equal depth from the surface of a silicon substrate so as to form foam so that the foam portion becomes a lacunose layer.
It is preferable to control the characteristics of the interface, by oxidizing the Si, which is exposed on the surface of the separation at the lacunose layer, at low temperature, or by oxidizing the same using O3 or H2O2, before the insulating layer forming process.
It is preferable to employ the plasma CVD method or SOG method (in which the oxide is solved into solvent, then is spin coated and finally the solvent is evaporated by a heat treatment), for forming the insulating layer. Because it is possible to carry out those methods without increasing the temperature so high. Thus, the electric components are not so influenced by the temperature.
It is preferable to form gettering cites previously in the semiconductor substrate so that impurities such as Fe or charges due to the ion implantation in the manufacturing process can be gettered.
The ultrathin semiconductor device according to the present invention comprises a semiconductor layer formed to have a thickness of below 15 micro meters, an electric component disposed on this semiconductor layer and an insulating layer formed at the backside of the semiconductor layer, and the total thickness from the insulating layer to the circuit lines layer on the front side of the semiconductor layer is below 20 micro meters. Such ultrathin semiconductor device can be obtained, for example, by following steps: forming electric components on a semiconductor substrate, which has an aforementioned lacunose layer at its surface side with respect to the lacunose layer; then separating the semiconductor substrate at the lacunose layer; and finally disposing an amorphous insulating layer, such as silicon oxide or silicon nitride, on the surface exposed by the separation, using the CVD method or SOG (spin on glass) method.
A three dimensional semiconductor device can be obtained, by putting on a plurality of such very thin semiconductor elements one over another. Namely a large scale integrated semiconductor device (LSI) comprising a large number of electric components but requiring small surface area can be obtained. Moreover, it is possible to obtain an ultrathin semiconductor device as a discrete electric component having an electrode at its backside, such as a large current transistor, by forming an electrode on the backside of the semiconductor layer.